If the new global variable hostzerobroadcast is zero, no longer assume
address zero of each net/subnet is a broadcast address.
(The default value is nonzero, which preserves the current behavior).
This can be set using sysctl; the boot-time default can also be
configured using the HOSTZEROBROADCAST kernel config option.
While we're here, defopt HOSTZEROBROADCAST and SUBNETSARELOCAL